The present invention relates to time-of-flight mass spectrometry, particularly to cryogenic particle detectors as ion detectors with charge discriminating capabilities in high-mass time-of-flight mass spectrometers, and more particularly to a cryogenically-cooled Nb-Al.sub.2 O.sub.3 -Nb superconductor-insulator-superconductor tunnel junction (STJ) detector which enables near 100% detection efficiency for all ions including single, very massive, slow moving macromolecules.
Time-of-flight mass spectrometry (TOF-MS) is a fast, inexpensive and efficient technique for characterizing macromolecules and is commonly used in biology and biomedicine to measure the mass of biological molecules. One prominent example for TOF-MS is the matrix-assisted laser desorption and ionization (MALDI) time-of-flight mass spectrometer. In a MALDI-TOF system the sample molecules are embedded in a light-absorbing matrix and are vaporized and ionized by a short laser pulse and accelerated by a high voltage (.about.30 kV). The molecular ions then fly ballistically through an evacuated flight tube of given length and their arrival at the other end is registered by a detector. Measuring the flight time of the molecular ions between the laser pulse (start signal) and the detector signal (stop signal) allows one to calculate the mass of the ions (more precisely, the mass/charge ratio).
Conventional mass spectrometers for biomolecules use microchannel plates (MCPs) to measure the arrival times of molecular ions. An ion impacting onto the front metal surface of the MCP can produce one or several secondary electrons which are then multiplied in the MCP and give rise to the signal, a short charge pulse. For large molecules (M&gt;50 kDa) the velocity attained in a typical mass spectrometer is too slow to produce secondary electrons efficiently on the surface of the MCP. Thus, the detection efficiency of an MCP drops dramatically for large masses in a typical TOF mass spectrometry system. The utility of existing MALDI-TOF-MS for studying large biomolecules is therefore severely limited by the lack of detector sensitivity at high masses. Thus, there has been a need for dramatically improving the sensitivity and the mass range accessible by MALDI-TOF-MS.
It has been found that the use of cryogenic detectors in TOF-MS systems solves the sensitivity problems associated with MCP detectors. Cryogenic detectors are a new class of very sensitive, energy-resolving, low-threshold particle detectors which respond to ion energy and do not rely on secondary electron production.
Cryogenic detectors are currently being developed for a variety of applications in particle and nuclear physics, such as x-ray spectroscopy, optical spectroscopy, and searches for dark matter in the form of weakly interacting massive particles (WIMPs).
Cryogenic detectors rely on measuring low-energy solid-state excitations as part of their detection mechanism, and therefore must be operated at temperatures typically below 2 K to avoid excess thermal excitations. The energy of these excitations, typically .+-.5 meV, is much less than the .about.eV energies needed to produce secondary electrons or electronic excitations in conventional ionization detectors, such as the MCP. Thus, a relatively large number of excitations is created for given energy deposition which allows the energy to be measured with smaller statistical error and thus much greater precision. This low excitation energy makes cryogenic detectors much more sensitive to weakly ionizing, slow moving particles than ionization detectors. Cryogenic detectors are therefore ideal for measuring the mass of large species, such as massive biomolecules, in time of flight mass spectrometry.
Another advantage of cryogenic detectors is that they are energy-resolving detectors, i.e., the measured pulse height is roughly proportional to the total ion energy. This can be exploited for TOF mass spectrometry in several ways. First, the energy resolution can be used to distinguish ions with different charge states. A doubly-charged ion carries twice the kinetic energy and will result in a pulse whose height is twice as large as that of a singly-charged ion accelerated by the same voltage. Charge discrimination is very valuable when ion launching techniques, such as electrospray, are used which create a large range of charge states making analysis with a conventional detector difficult. Charge discrimination is also useful for MALDI techniques, which generally produce a non-negligible fraction of multiply-charged ions, too. Second, good energy resolution may also allow details of the launching process to be studied by measuring the kinetic energy deficit or the internal energy large ions acquire during the launching and accelerating process in a TOF-MS system. Good energy resolution also may help to reveal where and how some of the macromolecules fragment in the TOF-MS system and thus assist in developing better TOF-MS systems.
There are various types of cryogenic detectors which offer both, high sensitivity to large molecules and good energy resolution, which can be used for charge discrimination. These include detectors based on the following sensors; superconductor-insulator-superconductor (SIS) tunnel junctions (often just called superconducting tunnel junctions or STJs), normal conductor-insulator-superconductor (NIS) tunnel junctions and transition edge sensor (TES). These sensors can be used as detectors just by themselves by directly bombarding them with particles or photons. To increase area and efficiency these sensors can also be coupled to a variety of larger particle or photon absorbers such as superconducting or normal conducting metal films, superconducting crystals or dielectric crystals. In addition, several sensors or sensor/absorber combinations can be grouped into arrays to increase the effective detector area.
SIS tunnel junctions consist of two layers of superconductors (S) separated by a thin insulating barrier (I), for example, Nb-Al.sub.2 O.sub.3 -Nb. When the tunnel junction is cooled to well below the critical temperature of the superconducting layers nearly all the conduction electrons form weakly bound pairs, called Cooper pairs. The binding energy of a Cooper pair is 2.DELTA.where .DELTA. is the superconducting gap and typically of the order of 1 meV or less. When a particle, such as a MALDI ion strikes the surface of an SIS tunnel junction, the kinetic energy of the ion creates non-thermal phonons (quantized crystal lattice vibrations) which are then absorbed by the superconducting films. In this process many Cooper pairs are broken up. As a result, so-called quasiparticle excitations are created which can then quantum-mechanically tunnel through the tunnel barrier producing a measurable current pulse when a small bias voltage of the order of 1 mV is applied to the junction. Since only a few meV are required to break a Cooper pair the kinetic energy of a MALDI ion, typically tens of keV, produces millions of quasiparticles. The magnitude of the tunneling current pulse is proportional to the number of quasiparticles produced which in turn corresponds to the amount of energy deposited into the detector by an impacting ion. The duration of the current pulse is given by the quasiparticle lifetime which is typically a few microseconds. The pulse onset corresponds to the MALDI ion arrival time and can be measured to .about.100 ns which is sufficient for most large-molecule MS applications. This time resolution may be improved in future versions of these STJ detectors optimized for MS applications.
Variations of this simple SIS tunnel junction are SIS' tunnel junctions and SIS or SIS' tunnel junctions with superconducting trapping layers. In an SIS' tunnel junction (also sometimes called a heterojunction) the two superconducting layers are made of materials with different superconducting energy gaps. Such junctions are used to study the behavior of tunnel junctions and for some special applications. The signal from an SIS or SIS' junction can be increased by adding a so-called superconducting trapping layer on one or both sides of the tunnel barrier. These trapping layers are made of superconductor with lower energy gap and serve to concentrate quasiparticle excitations near the tunnel barrier thus increasing the signal. One example of such a device would be a Nb-Al-Al.sub.2 O.sub.3 -Al-Nb junction. Typically STJs with trapping layers have larger signal and better energy resolution, but have to be operated at a lower temperature to avoid thermal quasiparticle excitation in the lower-gap trapping layers.
NIS tunnel junctions consist of one layer of normal conducting metal (N) and one layer of superconductor (S) separated by a thin insulating barrier (I), for example, Cu-Al.sub.2 O.sub.3 -Al or Ag-Al.sub.2 O.sub.3 -Al. Under proper bias conditions the tunneling current in such a device is a very sensitive function of the temperature of the normal metal electrode. Therefore, NIS tunnel junctions can be used as very sensitive thermometers. When a particle, such as a MALDI ion strikes an NIS tunnel junction or a normal metal absorber attached to an NIS junction the kinetic energy of the ion is ultimately converted to heat which briefly warms the NIS junction. The temperature rise is proportional to the deposited energy and can be measured as a tunneling current pulse.
Transition edge sensors (TESs) are another type of sensitive thermometers which can be used in the same way as NIS junctions to measure the impact of particles in a TOF-MS system. A TES consists of a thin film of superconductor which is operated in its transition from the superconducting to normal conducting state. In this transition region the electrical resistance of TES is a very sensitive function of temperature. The short temperature rise caused by the impact of a particle onto an TES or an absorber connected to a TES briefly changes the resistance of the TES and can be measured with the proper readout circuit as a current or a voltage pulse. TES sensors can be made either of pure superconductors such as Nb, Ta, Al, Mo, Zn, Cd, Ti, Ir and Hf or of bilayers or multilayers of normal metals and superconducting metals, e.g. Ag/Al, Cu/Al or Au/Ir. The addition of a normal metal film to a superconducting film results in the lowering of the superconducting transition temperature by means of the proximity effect. This is often done to lower the operating temperature and thus to increase the sensitivity of a TES based detector.
NIS and TES sensors, often also called "hot-electron microcalorimeters", are true thermal sensors measuring the heat ultimately generated in the detector by a molecule's impact. They are relatively slow (.about.30-300 .mu.s time constants) and have to be operated at very low temperature (.about.0.1 K or below) for best performance. As a potential advantage NIS or TES based detectors can cover an even better energy resolution than SIS tunnel junction based detectors. In contrast to NIS or TES based sensors, SIS tunnel junctions, or "STJ microcalorimeters", measure a non-thermal quasiparticle signal created by non-thermal phonons immediately after a molecule's impact before the deposited energy thermalizes and is converted to heat. Therefore, SIS tunnel junctions offer a higher speed and can be operated at a somewhat higher temperature (.about.1 K, depending on the superconducting material) than NIS or TES based detectors. The higher the operating temperature of a cryogenic detector the easier is its implementation into a time-of-flight system and the more room temperature thermal radiation the detector can be exposed to. Very small tin (Sn) STJ sensors have been utilized in a TOF system before this work. Compared to the Nb STJ sensors used in this work Sn STJ sensors require a relatively low operating temperature of 0.3 K, close to the typical operating temperature of NIS or TES sensors and thus already severely limiting the detector area which can be exposed to room temperature operation. Whether NIS tunnel junctions, TES sensors or SIS tunnel junctions are optimal and should be used for a given application will be determined by the actual requirements of a measurement.
For all types of detectors discussed here the signal can be increased by placing the detectors onto very thin substrates or membranes, made of a mechanically strong insulator, such as Si.sub.3 N.sub.4. When the detector is located on a membrane the phonons created by a macromolecule's impact are prevented from escaping from the vicinity of the detector. This increases the fraction of phonons absorbed in the metal layers of the detector and thus the measured signal height.
For all three types of the cryogenic detectors discussed here the detector area of existing prototypes is small, about 0.2-0.5 mm on a side, which is not ideal for MS applications. Increasing the size of an individual detector is possible, but usually results in a degradation of sensitivity, energy resolution and speed. To increase the effective area many individual detector elements can be grouped into larger arrays in which each individual detector element is read out by its own electronic channel. Since most cryogenic detectors can be fabricated by photolithographic techniques fabricating large arrays of detectors is almost as simple as fabricating a single detector.
Based on the recognition of the capabilities of cryogenic detectors for TOF-MS applications, the present invention is directed to the use of normal conductor-insulator-superconductor (NIS) tunnel junctions, transition edge sensors (TES), and superconducting tunnel junction (STJ) detectors in TOF-MS systems, and more particularly to a cryogenically-cooled Nb-Al.sub.2 O.sub.3 -Nb STJ detector for TOF-MS systems. Such a Nb-Al.sub.2 O.sub.3 -Nb detector has experimentally demonstrated the high detection efficiency of cryogenic detectors for high-mass biomolecular ions when used as a detector in a MALDI time-of-flight mass spectrometer. It can be operated at 1.3 K in a room temperature TOF-MS for large-biomolecules and cycled nearly infinitely. Thus, it has been demonstrated that by the use of the superior sensitivity of cryogenic detectors, slow-moving massive molecules can be effectively detected, that the energy resolution offered by such detectors can be utilized to measure and discriminate the charge of the ions and to study ion fragmentation. In addition to biomolecular ions, future applications may include other particles such as polymers, aerosol droplets and viruses.